In a conventional induction motor, it has been known that two notches are provided to an inner diameter part of a rotor iron core, the notches are used for a lubricant passage notch, so that a die-cast rotor can be produced precisely (refer to Patent Document 1, for example).
Further, in another conventional induction motor, an end-ring is elongated to an inner diameter area of the rotor iron core, and then a vent hole is formed on the end ring and the rotor iron core, so that mechanical strength can be improved without generating a nest inside the end ring (refer to Patent Document 2, for example).    Patent Document 1: JP59-10159    Patent Document 2: JP61-244248